Herein, we have designed and synthesized a quinolinyl-AIE photosensitizer (TPE-4QL+) with an alternative elevated intersystem crossing (ISC) rate, which exhibits not only highly efficient photosensitivity but also high tumor cell specificity and an excellent mitochondrial targeting ability. In vitro experiments indicate that using TPE-4QL+ as a photosensitizer can induce a series of tumor cells to die with a low dose of radiation, but with no obvious toxicity to normal cells. The in vivo studies on a mouse model bearing a subcutaneous 4T1 xenograft also show that TPE-4QL+ can be used with high efficiency as a photosensitizer in PDT.